Single crystal wafers, of which typical examples are those of silicon (Si) and gallium arsenide (GaAs), are obtained by slicing a single crystal ingot produced by the Czochralski method (CZ method) or the floating zone method (FZ method) into wafers. Therefore, it is desired to obtain wafers as many as possible from one ingot by making thickness of wafer as small as possible or reducing the stock removal for slicing. That is, desirability of reducing thickness of wafers or processing loss in the production of wafers to reduce waste of the raw material and thereby reduce production cost of wafers has hitherto been widely recognized.
For example, as an apparatus for slicing silicon wafers from a silicon single crystal ingot, a wire saw and inner diameter slicer are most generally used. However, since stock removal for slicing is required when silicon wafers are sliced by using these apparatuses, there are generated loss of the raw material. Even using a wire saw, which generates comparatively little slicing loss, slicing loss for about 200 μm per wafer is unavoidable. Further, the slicing by a wire saw, inner diameter slicer or the like leaves a damaged layer at a sliced surface after the slicing. Therefore, lapping, etching and so forth for removing the damaged layer are required, and they also generate loss of raw material. Furthermore, in such a mechanical processing, if a wafer is sliced with an unduly small thickness from the raw material ingot, it becomes likely that the wafer breaks during the processing. Therefore, a wafer must be sliced with a thickness larger than required and then processed to have a desired thickness by performing back lapping or back grinding after devices are finally produced. Therefore, a significant portion of the expensive single crystal material is wasted vainly.
Meanwhile, it has been also contemplated that the slicing of a single crystal is attained by laser processing, which is widely used for welding and cutting in the other fields. Laser processing generally provides higher precision compared with conventional mechanical processing and therefore it has advantages that precise processing is possible and waste of the raw material is little. However, the laser processing has a problem concerning melting due to evolution of heat or the like, and thus it suffers from disadvantages that peripheral portions under slicing are degraded and processing traces are remained. Therefore, it has been difficult to apply the laser processing to the semiconductor processing, which requires processing precision less than micrometer order.
To solve this problem, there has been advancing development of ultra short pulse lasers that can be employed for slicing of semiconductor single crystals. If the processing is performed by using an ultra short pulse of a pulse length of about several tens femtoseconds, excitation time is shortened to the same level as the relaxation time of atomic vibration, and bonds between atoms themselves can be cut without generating heat. Therefore, unlike the melting by evolution of heat, it does not cause degradation of peripheral portions under slicing and processing traces, and it may enables high precision processing of only the irradiated portion. Furthermore, if a short wavelength laser such as an excimer laser is used, photon energy is equal to the energy required to cut atomic bonds, and thus high quantum probability can be obtained. Therefore, high-speed and high efficiency processing becomes possible. At the same time, since a laser compressed into an ultra short pulse have an extremely large maximum power of the pulse, more effective processing is realized by nonlinear optic effects such as two-photon absorption.
However, a slicing method utilizing such an ultra short pulse laser has problems, for example, atomic substances removed by the laser slicing reattach t o the processed surface and so forth, flatness of the sliced surface cannot be obtained and thus processed form is degraded. Therefore, even if a laser slicing method utilizing an excimer laser or the like is applied as it is to the slicing of a single crystal such as a silicon single crystal, the advantage of the laser slicing that high speed and high efficiency processing is possible cannot be utilized effectively, and thus yield of the slicing of a single crystal cannot be improved compared with mechanical processing using a wire saw or inner diameter slicer.